Abstract Satellite cells are muscle-resident stem cells responsible for postnatal muscle growth, maintenance and regeneration. In response to muscle injury, quiescent satellite cells (QSCs) are activated, enter the cell cycle and proliferate, then differentiate to repair the injury or self- renew to replenish the satellite cell pool. Our long-term goal is to understand the molecular regulation of satellite cells and use this knowledge to improve the regeneration and function of skeletal muscles during aging or under pathological conditions. Compelling evidence demonstrates that deterioration of both systemic and cell-intrinsic properties result in reduced satellite cell function and accelerated muscle wasting during aging. The overall goal of this proposed study is therefore to explore novel molecular pathways that underlie the function of satellite cells. The phosphatase and tensin homologue (PTEN) is a dual-specificity lipid and protein phosphatase. PTEN was originally identified as a tumor suppressor mutated in many malignancies. Emerging studies have identified a role of PTEN in several adult stem cell types. However, the role of PTEN in satellite cells is completely unknown. This proposal aims to fill in this critical knowledge gap. Our preliminary results provide compelling evidence that PTEN KO in postnatal satellite cells leads to ablation of this stem cell population. The first aim will combine PTEN KO and lineage labeling to directly examine the fate of PTEN null satellite cells. This Aim will also determine how PTEN deletion in satellite cells affects muscle regeneration and gene expression. Interestingly, PTEN KO in embryonic myoblasts leads to postnatal muscle hypertrophy and better muscle function in young mice, but the long-term effect of this KO on satellite cells is unknown. The second aim will investigate the consequences of PTEN deletion in activated satellite cells and proliferating myoblasts, and age-dependent muscle function. The third aim will dissect the molecular mechanism underlying PTEN function in quiescent (Aim 1) and activated (Aim 2) satellite cells by identifying the upstream regulators and downstream effectors of PTEN in satellite cells. The molecular regulation of PTEN intracellular localization, the relative role of cytoplasmic and nuclear PTEN, and the interaction between PTEN and Notch signaling will provide new insights into PTEN regulation and function in stem cells independent of outcomes of Aim 1 and Aim 2. Together, this proposal will for the first time elucidate a role of PTEN in satellite cells.